Capacitive microelectromechanical system (MEMS) microphones are becoming increasingly important in various fields of application. This is essentially due to the miniaturized design of such components and the possibility for integrating additional functionalities at very low manufacturing costs. The integration of signal processing components such as filters and components for noise suppression, as well as components for generating a digital microphone signal, is particularly advantageous. Another advantage of MEMS microphones is their high temperature stability.
The diaphragm of the microphone structure is deflected by acoustic pressure. This causes the distance between the diaphragm and the stationary counterelement to change, resulting in a change in capacitance of the microphone capacitor. These very small changes in capacitance in the AF range must be converted into a usable electrical signal. The magnitude of the measuring signal is essentially a function of the level of the charging voltage of the microphone capacitor. However, a high charging voltage results in strong attractive forces between the movable diaphragm and the rigid counter electrode which promote electrostatic adherence of the diaphragm to the counter electrode. In the case of uninsulated electrodes, the contact of the diaphragm and the counter electrode results in a short circuit of the microphone capacitor, which may even cause the diaphragm and the counter electrode to fuse together. However, even if the electrodes are provided with a dielectric insulation layer, large-surface contact must be avoided, since the attractive force which acts between the electrodes, and thus the force necessary to separate the electrodes from one another, increases with the size of the contact surface.
Various measures are known in practice for avoiding adherence of the diaphragm to the counter electrode. One option is to provide a particularly stiff diaphragm suspension and/or to increase the distance between the diaphragm and the counter electrode, which already prevents contact of the two electrodes with one another. Both measures have an adverse effect on the sensitivity of the microphone.
In addition, it is known to provide small-surface stops, made of an insulating material, on the diaphragm and/or on the counterelement, so that the required separating force is relatively small. However, this protective measure involves increased manufacturing complexity.